The present invention relates to the extraction of energy from a heat source by means of a working fluid which is regenerated in the cycle, and more particularly to a power generating cycle which permits the extraction of energy from low temperature heat sources.
Generation of energy by expansion of a working fluid is limited by the temperatures at which heating and cooling sinks economically can be used in the regeneration of the working fluid. Pure or azeotropic (subcritical) working fluids condense and boil at essentially constant temperatures which further limits the power generating cycle, especially the ability of the cycle to utilize low temperature heat sources. In an effort to overcome such deficiencies, attempts at combining absorption/refrigeration principles in the power generating cycle have been proposed. Such proposals additionally utilize a dissolved working fluid in a solvent so that the working vapor condenses over a range of temperatures and boils from the media (working fluid plus solvent) over a range of temperatures. Such binary working fluid pair permits extraction of energy from a source and rejection to a sink over a wider temperature range than cycles that merely employ pure or azeotropic working fluids.
Representative proposals on this subject include Nimmo et al., "A Novel Absorption Regeneration-Thermodynamic Heat Engine Cycle", Journal of Engineering for Power, Vol. 100, pp 566-570, The American Society of Mechanical Engineers (October 1978) and U.S. Pat. No. 4,009,575 which propose to use potassium carbonate as the solvent and carbon dioxide as the working fluid in the power generating cycle. Such binary pair is heated by a heat source which vaporizes the carbon dioxide therefrom. The working vapor passes through a superheater, and thence to the turbine whereat its temperature and pressure are lowered for performing useful work. The turbine exhaust then goes to a direct contact absorber. The weak solvent solution from the vaporizer is passed to an intermediate heat exchanger, thence to a cooler, and finally into the direction contact absorber for chemically combining with the spent working vapor. The reconstituted binary solution then is pumped to the heat exchanger to heat exchange with the weak solution of potassium carbonate and thence to the vaporizer. Another proposal is that found in U.S. Pat. No. 4,346,561 which proposes the use of a binary ammonia/water pair. The power cycle claimed utilizes a plurality of regeneration stages wherein the working vapor is condensed in a solvent, pressurized, and evaporated by heating. The evaporated working vapor then passes to a next successive regeneration stage while the separated weak solution is passed back to the preceding regeneration stage. Interestingly, the cycle in FIG. 4 of this patent appears coincidental with the cycle discussed in the Nimmo et al. ASME publication, cited above. Yet another proposal is that of Nagib, "Analysis of a Combined Gas Turbine and Absorption-Refrigeration Cycle", Journal of Engineering or Power, pp 28-32, The American Society of mechanical Engineers (Jan. 1971) which proposes to utilize the exhaust gases from a gas turbine to operate a refrigeration unit. The refrigeration unit is used to cool the air prior to its entering the compressor. The reduction in compressor-inlet temperature is stated to result in an improvement in thermal efficiency of the combined cycle as well as an increase in the specific output.
While such proposals and others have been a step forward in the power generating field, much room for improvement exists.